程序代写代做代考 algorithm Network Layer

Network Layer

All material copyright 1996-2012
J.F Kurose and K.W. Ross, All Rights Reserved

George Parisis
School of Engineering and Informatics

University of Sussex

Network Layer 4-2

Network Layer

objectives:
v  understand principles behind network layer

services:
§  network layer service models
§  forwarding versus routing
§  how a router works
§  routing (path selection)
§  broadcast

v  instantiation, implementation in the Internet

Network Layer 4-3

v  introduction
v  virtual circuit and datagram networks
v  what’s inside a router
v  IP: Internet Protocol

§  datagram format
§  IPv4 addressing
§  ICMP, IPv6

v  routing algorithms
§  link state, distance vector
§  hierarchical routing

v  routing in the Internet
§  RIP, OSPF
§  BGP

v  broadcast routing

Outline

Network Layer 4-4

Network layer

v  transport segment from
sending to receiving host

v  on sending side
encapsulates segments into
datagrams

v  on receiving side, delivers
segments to transport layer

v  network layer protocols in
every host, router

v  router examines header
fields in all IP datagrams
passing through it

application
transport
network
data link
physical

application
transport
network
data link
physical

network
data link
physical network

data link
physical

network
data link
physical

network
data link
physical

network
data link
physical

network
data link
physical

network
data link
physical

network
data link
physical

network
data link
physical

network
data link
physical network

data link
physical

Network Layer 4-5

Two key network-layer functions

v  forwarding: move packets from router’s input to
appropriate router output

v  routing: determine route taken by packets from
source to destination
§  routing algorithms

Network Layer 4-6

1

2 3

0111

value in arriving
packet’s header

routing algorithm

local forwarding table
header value output link

0100
0101
0111
1001

3
2
2
1

Interplay between routing and forwarding

routing algorithm determines
end-end-path through network

forwarding table determines
local forwarding at this router

Network Layer 4-7

Network service model
Q: What service model for “channel” transporting
datagrams from sender to receiver?

example services for
individual datagrams:

v  guaranteed delivery
v  guaranteed delivery

with less than 40 msec
delay

example services for a
flow of datagrams:

v  in-order datagram
delivery

v  guaranteed minimum
bandwidth to flow

v  restrictions on changes
in inter-packet spacing

v  security

Network Layer 4-8

Network layer service models:
Network

Architecture

Internet

ATM

ATM

ATM

ATM

Service
Model

best effort

CBR

VBR

ABR

UBR

Bandwidth

none

constant
rate
guaranteed
rate
guaranteed
minimum
none

Loss

no

yes

yes

no

no

Order

no

yes

yes

yes

yes

Timing

no

yes

yes

no

no

Congestion
feedback

no (inferred
via loss)
no
congestion
no
congestion
yes

no

Guarantees ?

Network Layer 4-9

v  introduction
v  virtual circuit and datagram networks
v  what’s inside a router
v  IP: Internet Protocol

§  datagram format
§  IPv4 addressing
§  ICMP, IPv6

v  routing algorithms
§  link state, distance vector
§  hierarchical routing

v  routing in the Internet
§  RIP, OSPF
§  BGP

v  broadcast routing

Outline

Network Layer 4-10

Connection, connection-less service

v  datagram network provides network-layer
connectionless service

v  virtual-circuit network provides network-layer
connection service

v  analogous to TCP/UDP connecton-oriented /
connectionless transport-layer services, but:
§  service: host-to-host
§  no choice: network provides one or the

other
§  implementation: in network core

Network Layer 4-11

Virtual circuits

v  call setup, teardown for each call before data can flow
v  each packet carries VC identifier (not destination host

address)
v  every router on source-dest path maintains “state” for

each passing connection
v  link, router resources (bandwidth, buffers) may be

allocated to VC (dedicated resources = predictable
service)

“source-to-destination path behaves much like
telephone circuit”
§  performance-wise
§  network actions along source-to-destination path

Network Layer 4-12

VC implementation
a VC consists of:

1.  path from source to destination
2.  VC numbers, one number for each link along path
3.  entries in forwarding tables in routers along path

v  packet belonging to VC carries VC number

v  VC number can be changed on each link

§  new VC number comes from forwarding table

Network Layer 4-13

VC forwarding table
12 22 32

1 2
3

VC number
interface
number

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 22
2 63 1 18
3 7 2 17
1 97 3 87
… … … …

forwarding table in
northwest router:

VC routers maintain connection state information!

Network Layer 4-14

application
transport
network
data link
physical

Virtual circuits: signaling protocols
v  used to setup, maintain teardown VC
v  used in ATM, frame-relay, X.25
v  not used in today’s Internet

1. initiate call 2. incoming call
3. accept call 4. call connected

5. data flow begins 6. receive data
application
transport
network
data link
physical

Network Layer 4-15

Datagram networks
v  no VC setup at network layer
v  routers: no state about end-to-end connections

§  no network-level concept of “connection”
v  packets forwarded using destination host

address

1. send datagrams

application
transport
network
data link
physical

application
transport
network
data link
physical

2. receive datagrams

Network Layer 4-16

1

2 3

Datagram forwarding table

IP destination address in
arriving packet’s header

routing algorithm

local forwarding table
dest address output link

address-range 1
address-range 2
address-range 3
address-range 4

3
2
2
1

4 billion IP addresses, so
rather than list individual
destination address
list range of addresses
(aggregate table entries)

Network Layer 4-17

Destination Address Range

11001000 00010111 00010000 00000000
through
11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000
through
11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000
through
11001000 00010111 00011111 11111111

otherwise

Link Interface

0

1

2

3

Datagram forwarding table

Network Layer 4-18

Longest prefix matching

Destination Address Range

11001000 00010111 00010*** *********

11001000 00010111 00011000 *********

11001000 00010111 00011*** *********
otherwise

DA: 11001000 00010111 00011000 10101010

examples:
DA: 11001000 00010111 00010110 10100001 which interface?

which interface?

when looking for forwarding table entry for given
destination address, use longest address prefix
that matches destination address.

longest prefix matching

Link interface

0

1

2

3

Network Layer 4-19

v  virtual circuit and datagram networks
v  network layer service models

Summary